Tables of Tarnish resistant sterling properties

I’m getting darn curious to see the trade offs between the differeing
sterlings. As a gold and palladium alloy user/developer/victim
(victim of my own errors sometimes) all these low or no copper
sterlings appear to cast very soft unless addtional steps are taken.
The harder ones (tell me if I’m wrong here) seem to have less tarnish
resistance compared to the softer ones. Some of my customers want a
"copper sterling" hardness and tarnish resistant sterling for models
intended for vulcanized rubber mold production. Real resistance to
oxides and sulphides would be perfect!

Is there a tarnish resistant sterling that casts at the same or
greater hardness than copper sterling with little or no extra work?
Inquiring cad-cam folks need to know…

Argentium, Platinum Sterling, S-57, are any of you up to this

Daniel Ballard